A Global Positioning System (GPS) receiver may be implemented in a handheld wireless communication device, such as a cellular phone. For example, United States Patent Application 2006/0250202 to Park et al. entitled “Apparatus and method for receiving GPS signals in a mobile terminal” is directed to an apparatus and method for increasing GPS reception sensitivity in a mobile terminal having a mobile-based GPS positioning function. A GPS receiver in the mobile terminal receives sensitivity assistance (SA) data by using basic information about GPS satellites and system time. The GPS receiver then correlates a PRN code with a GPS signal and coherent-integration of the correlation result by using the SA data. It calculates a position of the mobile terminal by using the integrated samples.
In the Enhanced 911 (E911) program, the GPS should be able to determine the location of the cellular phone even when a user makes emergency calls in an indoor environment, where the GPS receiver does not have view of the sky. High sensitivity is required for such an application, such as −155 dBm or higher.
United States Patent Application 2004/0102165 to Bloebaum et al. entitled “Compensation for frequency adjustment in mobile communication-positioning device with shared oscillator” discloses a method for compensating for a frequency adjustment in an oscillator shared between a communication circuit and a positioning signal receiver. In one embodiment, the method begins to receive and store a positioning signal at a first time point. When, at a second time point, the operating frequency of the shared oscillator is adjusted, the frequency adjustment is recorded. After the positioning signal is completely received and stored, the processing of the positioning signal takes into consideration the frequency adjustment. In that embodiment, the processing hypothesizes a frequency shift in the received positioning signal. In another embodiment, the method for determining the operating frequency of the oscillator detects a beginning time point of a reference signal received by the mobile communication device and enables a counter to count in step with a clock signal derived from the oscillator. When an ending time point of the reference signal is received by the mobile communication device, the count is stopped, and the frequency of the oscillator is determined based on the count in the counter and an expected time that elapsed between the beginning time point and the ending time point.
Achieving high sensitivity, a GPS receiver will try to correlate the local pseudo noise (PN) with the received signal using longest possible coherent correlation length. Coherent correlation length is limited by the phase noise of the frequency and time reference of the GPS receiver, which is usually implemented as a TCVCXO (temperature compensated and voltage controlled crystal oscillator). A low phase noise GPS TCVCXO may be critical to sensitivity performance.
The cellular transceiver usually also has a TCVCXO, that is controlled by an AFC (automatic frequency control) loop. The loop continually adjusts the control voltage of the cellular TCVCXO to maintain low frequency error that meets requirements of the cellular transceiver.
When the GPS receiver and the cellular transceiver share a single TCVCXO, the conventional AFC loop will usually make the phase noise of the TCVCXO too high for use of the GPS receiver, causing loss of GPS sensitivity.